1. Field of the Invention
This invention relates generally to the use of electricity for hot machining. More particularly, the invention relates to improved metal-working tools which use electrical current to heat the shear zones of metallic work pieces.
2. Description of the Prior Art
It is well known to those in the metal working art that metals and metal alloys tend to deform or shear more easily when heated. Since such deformation occurs in both the workpiece and the cutting tool, most metal-cutting operations take place at room temperature to prevent premature deterioration of metal-working tools. The difficulties encountered in machining high-strength materials such as titanium and inconel, however, have kindled renewed interest in hot machining processes.
A number of experimenters have investigated hot machining techniques involving the passage of electrical current through a workpiece for heating purposes while attempting to avoid rapid tool deterioration caused by such heating. U.S. Pat. No. 416,873 issued to B. C. Tilghman in 1889, a method is described whereby electricity is passed from a rotating cutting disk to a workpiece in order to soften the portion of the workpiece operated on by the cutting disk (the shear zone). In Tilghman's invention the cutting edge is connected to one electrode of a current source and the metal workpiece to the other, thereby causing current to pass between the cutting edge and the workpiece. The Tilghman method utilizes a rotating disk as the cutting tool in order to continuously change the point on the cutting edge through which current passes. This helps to minimize heating of the tool. However, when the metal-working tool is of other geometry such as a lathe insert or a milling tool or the like, a serious problem is created because electrical current passing through the tool cutting edge causes excessive heating and thus more rapid deterioration of the cutting edge. In 1962, Wennberg, Mehl and Krobacher published "Hot Machining of High Temperature Alloys Can Increase Production" in Volume 70 of SAE Transactions . The authors described various means for heating workpieces, such as resistance heating by passing electrical current either through the workpiece or through resistance heaters imbedded in the workpiece and Radio-Frequency (RF) resistance heating wherein RF energy was passed from a conventional chipbreaker to the chip being removed from a workpiece. With regard to the RF heating process, the authors stated, at page 152:
"Since the high current in the immediate vicinity of the tool heats the (cutting) insert and tool holder by induction, nonmagnetic materials must be used for the tool holder and the cutting tool . . . . Since all carbide tools are magnetic, an oxide tool was used in these tests. However, since the oxide tool is a good insulator, if the chip loses contact with the chipbreaker, the current arcs causing immediate shattering of the tool . . . . " PA1 1. higher metal removal rates; PA1 2. less power consumption; PA1 3. improved tool life; PA1 4. better surface finishes; PA1 5. improved size control; and PA1 6. ability to machine harder materials. PA1 1. lower metal removal costs; PA1 2. reduced cutting tool costs; PA1 3. less expensive holding fixtures; PA1 4. elimination of secondary grinding operations; PA1 5. less material distortion; PA1 6. cheaper machines for the same job; PA1 7. faster manufacturing-cycle times; and PA1 8. reduced maintenance costs.
The authors apparently did not consider through-the-tool D.C. current heating.
Other experimenters have reported experiments using electrical through-the-tool techniques. In Barrow, "Machining of High Strength Materials at Elevated Temperatures Using Electrical Current Heating", Annals of the C.I.R.P, XIV, Pages 145-151 (printed in Great Britain, 1966), the author describes electrical through-the-tool resistance heating techniques with a view to studying the effect of temperature on tool wear. Experimentally, a large alternating current (up to 500 A) was passed through the tool metal working edge to the workpiece. A disadvantage cited was that since the heat is generated at the tool (metal-working edge)/chip interface, the life of the tool is less than with previously used workpiece heating techniques.
In a later published article, Barrow, "Use of Electric Current for Hot Machining of High Strength Steels", Machinery and Production Engineering, Mar. 5, 1969, pp 370 et seq., the author described the use of both AC and DC electrical current of up to 1000 A in through-the-tool electrical heating techniques wherein the current was, once again, passed through the tool cutting edge to the workpiece. The essence of Barrow's results is that manipulation of current intensity, tool forces and cutting speeds can produce an improved tool life but that such improved life is limited by the heat generated by passing current directly through the tool metal-working edge (p. 371).
Thus, it is well established that through-the-tool passage of electrical current for hot machining may result in increased machinability of the workpiece. This technique enables the workpiece to be machined using less applied force than would be otherwise necessary, thereby increasing tool life. It also greatly reduces the vibration experienced by the cutting tool relative to that of the workpiece. Such hot machining allows high-strength metals to be worked more easily. The major disadvantage of this technique is that the improvement in tool life is limited by the heating and thus softening of the tool metal-working edge caused by electrical current passing therethrough.